This invention relates to ignition plugs for internal combustion engines, and more particularly to those provided with an ionization current detector electrode.
FIG. 1 shows a conventional ignition plug for an internal combustion engine capable of detecting the ionization current, and FIG. 2 shows the ignition circuit provided with a conventional ignition plug capable of detecting the ionization current.
The ignition plug 100 for an internal combustion engine includes a central electrode 1 and a grounded electrode 2 opposing the central electrode 1. The spark is generated across the central electrode 1 and the grounded electrode 2 to initiate combustion of the fuel/air mixture within the cylinder of an internal combustion engine. The central electrode 1 is covered with an electrical insulator 3. The electrical insulator 3 is partially surrounded by a housing 4. The central electrode 1 is coupled to an electrically conductive terminal 5, which is coupled to an ignition coil 7 via a diode 6 for preventing reverse currents. Further, the terminal 5 is coupled to the negative electrode of a DC voltage source 10 via a diode 8 for preventing reverse currents and via a resistor 9. Further, the cathode of the diode 8 is grounded via a capacitor 11 and a resistor 12. The junction point between the capacitor 11 and the resistor 12 is coupled to an output terminal 13, via which an ionization current detection signal D is obtained.
As shown in FIG. 2, the secondary side of the ignition coil 7 is coupled to the collector of a power transistor 15, the emitter of which is grounded. The base of the power transistor 15 is coupled to an input terminal 16 to which a control signal from a fuel injection timing/ignition timing control unit (not shown) is input.
The operation of the above conventional ignition plug circuit for an internal combustion engine is as follows. A high negative voltage I is generated by the ignition coil 7, which is applied via the diode 6 to the central electrode 1 of the ignition plug 100. Thus, a spark is generated across the central electrode 1 and the grounded electrode 2 to ignite the fuel/air mixture within the cylinder of the engine. The resulting combustion induces ionization of the gas within the cylinder and an ionization current is thus detected at the output terminal 13 as the ionization current detection signal D.
The above conventional ignition plug circuit has the following disadvantage. First, since two diodes 6 and 8 are utilized for separating the ignition voltage I and the detection signal D, the production cost is increased. Further, since a single cable 14 utilized for the ignition voltage I and the detection signal D constitutes a common impedance for both, the noise generated at the ignition makes the detection of the ionization current difficult.